Carrier-current signal system



H. AFFEL CARRIER CURRENT SIGNAL SYSTEM Augo ma 19%.; www

Filed Jan. 27, 1922 3 Sheets-Sheet 1 INVENTOR.

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ATTORNEYS. I

Aug i 9 192%o H. A AFFEL.

CARRIER CURRENT SIGNAL SYSTEM Filed Jan. 27, 1922 3 Sheets-Sheet 2 :1 BFm 14-15000 A} 5 m0 R w m lfatented r tngm l0,

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application filed January transmission over a given line, and separatinga; frequency band into narrower hands and transmitting them separately.These and other objects of my invention he made apparent onconsideration of a few specific examples which I have chosen forpresentationin this specification. it will he understood that thefollowingdescription refers to thseexamples and that the invention isdefined in the appended claims.

The drawings are diagrams. The symholr i iwill he understood to meancycles per second. Figure 1 shows a system for shiftmga frequency handof 5000 7000- to aloand of 300015000-. Figs. 1 and i are diagrams ofwave; filters. Fig. 2 shows a two-way repeater system with a moderatefrequency shift between the input and the output. Fig. 1i shows atwo-way repeater.

system with a frequency inversion between the input and output. Fig. 4shows a repeater system between two parallel cir cuits on one side and asingle circuit on the other side, the single circuit carrying a totalfrequency range corresponding to douhle the range of one of the twoparallel circuits, and Fig. 5 shows a system for carrying a voicefrequency range of 02000- over five lines, each carrying 0-400.

Referring to Fig. 1, this shows a one-way system by which alternatingcurrents of various frequencies from 50O0- to 7000- are shifted [tocorresponding frequencies lying between3000 and 5000 The modulator M.modulates 20000- frequency from the generator E with 1000 frequencyfrom the generator 1%. This modulator h/ 1 comprises two three-electrodevacuum tubes connected as shown in the diagram. This connection servestoeliminate the carrier frequency (namely 200005 from the output of themodulator system. The output frequencies from the modulator M} willaccordingly he l,-000-, i0000- and These go to a .highpass filter whosecritical-frequency is 5000-, which shifting a frequency, hand to. arange of values most'suitahle for ices. serial no. traces.

accordingly suppresses the l000- frequency. This high pass filter ofcritical frequency 5000- helongs to-the general type of filters shown inFig.1 having series impedances Z and shunt irnpedances Z in alternation.When impedance Z is represented physically by a condenser C andimpedance Z is represented physically hyj'an inductance L, the filtertakes the form shown in Fig. 1 and becomes a high pass filter, withcritical frequency @ther well-known combinations give the low passfilter and the hand pass filter. The remaining frequencies passed, bythis filter ofrcritical frequency 5000- are separated into respectivebranchconductorsi hy the two filters of critical frequency 20000-, oneheingj low-pass and the other high-pass.

Accordingly, the 10000- frequency goes.

over the conductors as shown in Fig. i, as the carrier current for themodulator M and the 2l000- frequency rier current for the modulator MThe 19000- frequency in the modulator:

goesas the oarll/lg is modulated by the input currents or variousfrequencies from 5000 to 7000'.

Accordingly, the output of the modulator ll I2 will be made up offrequency bands of .50U07000-, l2000l4000-, and 2l000 2eooo-. V

This composite output going to the highpass filter whose criticalfrequency is 15,000-, the output of this filter will he only the lastmentioned hand, viz, 24000 to 2fi000w. This frequency range is appliedto modulate the 2l000- frequency in the modulator M and accordingly inthe output from modulator M -there will he found the three frequencyhands 3000-5000 24000-26000 and l5000l7000-. This composite output goesto the low-pass filter whose critical frequency is l5000-, andaccordinglythe final output is only 3000 to 5000' it will be seen thatthe output frequency hand 3000 to 5000- lies adjacent to the inputfrequency band 5000 to 7000-. Further, it will be seen that this shiftof a frequency band to an adjacent frequency hand is accomplished byshifting first to a distant hand and then back to the adjacent hand.More specifically, the band 5,000

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7,000- is shifted to 24000-26000-, which is thenshifted back to30005000-.

If it had been attempted to make the shift directly from the inputfrequency band to the output frequency band, that is, from" obtained. Itwill be seen that the general plan is to modulate a 'carrier frequencylarge in comparison with the input frequency or band of frequencies, andthen to demodulate by asecond carrier whose frequency is equal to thatof the original carrier plus or minus the net frequency displacementwhich it is desired to effect. In other words, it may be said in thiscase that the most effective method of producing a small shift of afrequency band is to make a large shift and then asoi newhat greater orsmaller shift in the opposite direction, resulting in a differentialsmall shift, as desired. In this way, at

the various steps of modulation, the by-prodnot harmonic terms andundesired side bands are readily separated out by suitable filters,siiiee thevarious ranges of frequencies involved at each steparesubstantially nonoverlapping and non-adjacent.

The method of Fig. 1 will be seen to involve the local generation of twbcarrier currents differing moderately in frequency, viz, in thisparticular case, '1-9000 and;2lO00-.

These are generated by generating the intermediate or average frequency,viz,

2000O-, and modulating it by a frequency equal to half the differencebetween the two desired frequencies. In this case it will be seen thathalf the difference between 19000- and 21000- is 1,000-. Even if thereshould be a little variation in the frequency of the generator F so thatits fre uency varies a little from 20000-, neverthe ess, if thefrequency of the generator F remains constant at 1000-, the two outputfrequen-' cies 19000- and 21000- may vary a little but will keep theconstant difference of 2000-."

Referring toFig. 2, this illustrates a. two i way .device by whichalternating currents of 40006000-, coming from the west, are shifted to30005000-, going east, and on the other hand, alternating currents of3000-5000- coming from the cast are shifted to 40006000- going west. Theoperation of the unit shown in Fig. 2 will be readily understood fromthediscussion that has been substantial currents in the other.

given of Fig. 1. Input eurrents.from the west pass to the primary of thethree winding transformer T of well known design whose input and outputbranches are conjugate, so that currents in one such branch produce noThe cur-. rents from the west are assumed to lie in the range 40006000-and go through the two band filters whose transmitting ranges are 4000to 6000-,, but these currents are blocked by the two band filters whosetransmitting ranges are 14000 to 16000-. Accordingly, these frequencies.4000 to 6000 go V respectively to the audion grids and plates of themodulator M But the audions being unilateral devices, the effect on theplates of the frequencies considered is nil. The modu lator M isconnected with a 10000- cycle generator and the frequencies 40006000- onthe grid side are modulated with the frequency 10000-, so that theoutput of the modulator M comprises the frequencies 10000-', 4000600 0'-and l400016000-..

Of these ranges, only that of 40006000- is passed to the west by theupper left band filter, and only the last range, namely 14000-16000-, ispassed to the east through the upper. middle band filter'of Fig. 2. Thisfrequency range 1400016000- is blocked I by the upper righthandband'filter of range 3000-5000 input of the modulator M where it iscombined with the locally generated frequency 11000-, giving an outputof 1l000-, 30005000-, and 25000.27000-. All of these ranges are blockedby the lower middle band filter of range 14000,16000-. Only the30005000- band .is passed by the lower righthand band filter of Fig. 2.frequency range is applied at the three-winding transformer T at theneutral points so that it goes equally to the line to the east and toits balancing network, and not at all into the secondary circuit.Accordingly it will be seen that the 40006000- range from the west isrepeated to the east as a 3000:5000- range. The full-line arrowsindicate the course of the essential currents through the apparatus forthis repeating operation. The dotted arrows indicate the repeatingoperation from east to west by which incoming currents of a frequencyrange 3000-5000 are transformed to output 'currents of a range'40006000-. The repetition from east to west is the same in principle asfrom west to east and will be understood without.

further description.

Thus it will be seen that any frequency entering the circuit from\ theeast within the range of 3000 to 5000- will appear in the west increasedby 1000- and reciprocally, any frequency entering from the west withinthe range 4000 to 6000- will appear in the east decreased by 1000-.'

A use for a unit'such asthat shown in Fig. 2 is toinstall it at the endsof alineover This Accordingly it goes to the 1 stations will make whichit is desired that telephone transmisslon shall besecre't; thus, insteadof an ordinary repeater station which repeats without changing thefrequency, the repeater-Sta tions of Fig. 2 may be installed at the endsof astretch of line. Assuming that voice frequency currents are put inat one end and are shifted by the device, their altered frequency on thestretch between the two them unintelligible for direct reception.

It would be still more effective to invert thefrequencies and a unitthat accomplishes this purpose is shown in Fig. 3. It will be seen thatin general structure the arrange ment is the same as for Fig. 2.".Whereas in Fig. 2, the band filters transmitting from H000 to 16000-are transmitting an upper. side band, in Fig. 3 theband filterstransmitting from 8000 to 10000- are transmitting a lower side band. Theresult isv that the frequencies are inverted. For example, a frequencyof 500 from the westgoes to the modulator M and appears in its output asa component, 95006 of the lower side band which goes through the bandfilter transmitting from 8000 to 10000-. This 05500- frequency iscombined in the modulator M; with the locally generated frequency 8000-and-gives 1500- in the lower side band of the output of modulator M Thisis passed by the lower righthand band filter whose range is 0 to 2000-and appears in the line eastas a frequency of 1500-. ln' short anyfrequency nbetween 0 and 2000- in the line west gives an outputfrequency of (2000n)- in the'lineeast, and vice versa. Accordingly, thesubstitution of two repeater stations like Fig. 3' for the ordinaryrepeater stations in a long transmission line secures the result thatbetween those stations all the voice currents will be of invertedfrequency and quite unintelligible for direct reception. 7

lt has been suggested to attain secrecy by transmitting with the voicecurrents a pure tone loud enough to drown out the voice and to separatethis tone from the voice currents at the receiving end. However, it isdifficult to separate out the pure tone lying within this frequencyrange without suppressing a whole range of adjacent frequen cies andseriously distorting the voice currents. By my method for example, apure tone of lO0- might be put on the line with the voice currents sothatanywhere on the line, by direct reception, this tone would competelymask the voice currents. At the 0 receiving station the receivedcurrents would be displaced down 400 thus causing the 400- loud tonecurrent to appear at approximately 0-.' it could then beeasily filteredout and the frequency range shifted back by 400 giving the voicefrequency range substantially unimpaired as transmitted at the outset.

indicate the resultant frequency shift.

It is known that loaded cables have a tendencyrto attenuate highfrequencies relatively more than low frequencies, so that transmissionof a wide frequency range over a loaded cable may be impractical on thisaccount. A much wider range can be trans I mitted on an. open wire linebecause the at.- tenuation does not fall ed with increasing frequency sorapidly in that case. It is common that a long distance transmissionwill' be partly by cable in a region of dense trafiic and partly by openwire in an outlying region. The direct connection of a cable in serieswith an open wire therefore cuts down the capacity'of the open wire tothe frequency range which may be transmitted over the cable. lnFig. 4,1have shown an arrange ment by which one open wire line can be made toserve for connection in series with each of two cables. This figureillustrates a carrier current telegraph system with s'm, channels eachWay on an open wire line connected serially to each of two cables, eachcable having hree channels corresponding to three of the six of the openwire line. In Fig. 4, the rectangles marked (Fig. 1) represent thesystem of Fig. 1 by which oneway frequency shift is obtained. Therespective numerals in these rectangles,-viz, 300-, 12o0-, +1500-, and+3400 11 this connection it will be understood that the correspondingnumeral for Fig. 1 would be 2000-. It will be seen that from west toeast on the open wire line there are six carrier current frequenciesranging by 300- intervals from 3300 to 4800-. These are separated intotwo groups by the band filters BF and BF Then the frequencies areshifted down as indicated and as already mentioned, and each group ofthree carrier frequencies is put on a respective cable, each groupcomprising 8000, 3300 and 3600-. Other frequencies namely 4000, 4300 and4600 come in over the cables from east to west and go through the bandfilters BFg and BF; to the devices like Fig. l, where the frequenciesare raised by 1500- for the group from cable No. 1,"and by 3400- for thegroup from cable No. 2. This gives the set of six west-bound carrierfrequencies, ranging by 300 cycle intervals, from 6500 to 8000 Thus itwill be seen that the greater frequency range of the. open wire line ascompared with the cable is utilized by making one open wire line'servein series with each of a plurality of cables.

. In cases for which it'is desired to transmit voice currents over aline of limited frequency range, my invention may be employed to dividethe voice range up and apportion the parts thereof to conductors eachhaving a frequency range suflicient to carry its respective portion. ltis well known that transmission through ocean cables is possible onlyfor the relativelyv low frcquencies involved in telegraphy and not forthe higher frequencies involved invoice currents. Referring to Fig. 5,sup ose that there are five lines, L L L L an L each 5 of which willcarry effectively. a frequency range of only 0 to 400-, and that it isdesired to transmit between their common terminals a voice-frequencyrange of 0 to 2000-. At one end a set of five band filters will separatethe. voice frequencles into five consecutive bands, each of range 400.These various frequency ranges will then be'depressed by the devices ofFig. 1, and by amounts indicated by the numerals -400-, -800-, etc., sothat the output frequency ranges for each of the five devices will befrom 0 to 400-. These ranges will be transmitted over the fiverespectivelines and at the other end the respective frequency rangeswill be increased by +400 +800-, etc.,and 'thenrecombined to give acorrect voice frequency range.

With the simplest types of carrier modulating circuits difficulty hasbeen experienced .using carrier frequencies lower than about 4000-. Inthis case, the harmonics of the .voice frequency ange, produced by thenormal action of the modulating circuits, are transmitted in the carrierband itself, 4,000 to 6,000-, and cause effective noise and qualitydistortion in the demodulating circuit at the receiving end. For suchlow carrier frequencies, and even lower carrier frequencies, whendesired, "it will be seen that my system will be advantageous. Such lowcarrier frequencies might be useful for loaded cable. nel might beoperated at say 2500- for the carrier frequency. in addition to a voicerange channel.

The amount of amplifying gain at voice frequencies is often limited bythe incidental noise produced in the vacuum tube circuits, as, forexample, by the vibration of the vacuum tube grid. By introducing theamplification in the high frequency part of the circuit much of thistrouble may be avoided. 'Accordingly, it may be desirable to shift thefrequencies to a high frequency merely to secure this advantage in cases0 where no ultimate frequency displacement is desired.

I claim:

1. The method of producing a modula tion of a comparatively lowfrequency by a 55 range of frequencies Without intermediatetransmission, which consists in modulating a comparatively high frequencby such range and then directly demodu ating by a frequency differingfrom said high frequency by the desired frequency.

2. The method of shifting a frequency band at a single station by acomparatively small frequency displacement, which consists in firstshifting it by a wide frequency displacement and then at the samestation A carrier telegraph chan memes shifting it back to the desireddegree 0 displacement.

3. The method of producing a comparati vely small displacement of afrequency band, which consists in shifting it by modulation over acomparatively wide displacement, then filtering. out undesired frequencybands, then directly at the same station shifting back to the desiredpoint of displacement by another modulation, and againfiltering outundesired frequencies.

4. The method of shifting the frequency range of a wave by acomparatively small frequency displacement, which consists in modulatingit with one comparatively large frequency and then demodulating withanother frequency differing therefrom by the ultimate displacementdesired, and generating the two modulating frequencies by modulatingtheir average with half their difference.

5. The method of repeating a frequency range which consists in shiftingsuch range by a comparatively large frequency displacement, anddirectlyat the same station shifting back by a range differing slightly from thefirst range, and'transrnitting the resultant frequency range.

' 6. A two-way repeater comprising means both to transmit and receivethroughout one "frequency range on one side, means both to transmit andreceive throughout one different frequency range on the other side, andmeans to shift the frequency up one way and down the other way .fromone'said range to the other, the said two ranges being near to-' getherin frequency, so as to effect transmission on each side each way withinand throughout the respective frequency ranges.

7. A two-way repeater comprising two three-winding transformers, twoopposite modulators, one to step a frequencyup by a certain number ofcycles and the other to step it down. by a number of cycles slightlydifferent, respective generators of different frequency for these twomodulators, and filters connected for two-way transmission whereby thetransmission on one side of the repeater both ways is' at a slightly.different frequency than on the other side of the repeater both ways.

8. A two-way repeater comprising two three-winding transformers, twoopposite modulators, one to step a frequency up by a comparatively widefrequency range and the other to step it'back by a range nearly but notquite the same, respective generators for the two modulators each offrequency high compared to the frequencies in said transformers but thetwo generators being of slightly different frequency, and filtersconnected for two-way transmission whereby the transmission both ways onone side of the repeater is at a frequency slightly different from thetransmission both ways on the other side of the repeater.

til

lit

r ea-rec either transformer to a comparatively distent range and thenback not far from the original-range but inverted at the othertransformer. v7

11. The method of transmitting a band of waves over a conducting pathwhich ofiers an excessive attenuation to the higher frequency componentsthereof, which method comprises reducing the frequencies of allthecomponents to frequencies well the eficient transmission range ofsaid path.

12. The method of transmitting a band of waves lying between twodefinite frequency limits over a conductor within the transmission rangeof which one of said limits does not fall, which method comprisesreduc-- mg the frequencies of each component of.

said band by a predetermined amount to cause said frequency limits tofall within said transmission range, transmitting said reduced frequencyband over said conductor and thereafter increasing the frequency of eachof the transmitted components by the same predetermined amount bywhich'it was reduced before transim'ssion.

13,, The method of transmitting between two points aband of waves ofbroader frequency extent than the transmission rangeof a practicaltransmission circuit which can be used to connect said points,comprising separating said band into a plurality fill of nonoverlappingfrequency sub-bands of narrower extent, transmitting each of saidsub-bands over an individual transmission circuit, connecting said twopoints, receiving said sub-bands, and re-assembling them to produce saidoriginal band of waves. 7

M. The method of transmitting between two points a band of waves ofgreater fre quency extent than the practical transmission range of acircuit thatmay housed to connect said points, comprising dividing saidband into a plurality of non-overlapping frequency bands, reducing thecomponent frequencies of each of said v plurality of bands, transmittingeach of said reduced frequency hands over an individual circuitconnecting said points, restoring each of said transmitted bands to itsoriginal frequency position, and re-assembling said restored frequencybands to reproduce the original band.

15. The method of transmitting over a conductor waves having componentshigher than the practical upper frequency transmission limit of saidconductor, which com prises so combining said waves with a wave of asingle frequency as to reduce the component frequencies by said singlefrequency, transmitting said wave of reduced frequency components oversaid conductor, and thereafter so combining said transmitted wave with awave of said single frequency as to increase the component frequenciesof the -transmitted wave to their original magnitudes. v

16. A. transmission system for transmitting a band of waves over. aconducting path which ofiers excessive attenuation to the of elicit thecomponentsof said band to frequencies well within the emcienttransmission range of said path, and transmitting said reduced frequencycomponents over said path.

l'Z.- A system for transmitting a band of waves lying between twodefinite frequency limits over a conductor within the transmission rangeof which one of said limits does not fall, which comprises means forreducing the frequency of each component of said band by" apredetermined amount to cause said frequency limits to fall within saidtransmission range, means for transmitting said reduced frequency bandover said conw ductor to a distant station, and means at .said stationfor increasing the frequency of each of the transmitted components bythe same predetermined amount by which it was reduced beforetransmission.

18. A system for transmitting between two points a band of waves ofbroader frequency extent than the practical transmission range of acircuit which can be used to connect said points, comprising means forseparating said band of waves into a plurality of non-overlappingfrequency sub-bands of narrower extent, an individual transmissioncircuit for each of said sub-bands connecting said two points, means forsupplying each sub-band to its individual transmission cir fill higherfrequency components thereof, comprismgmeans for reducing thefrequencies rot cuit, means for receiving said sub-bands at a distantpoint, and means for re-assembling them to produce said original band ofwaves. 19. A system for transmitting between two points a band of wavesof greater frequency extent than the practical transmission range of acircuit that may be used to connect said points, comprising means fordividing said band into a plurality of nonoverlapping frequencybands,.means for reducing the component frequencies of each of saidplurality of bands, means for trans mitting each of said reducedfrequency hands over an individual circuit, means for restoring each ofsaid transmitted bands to its original frequency position, and means forre-assembling said restored frequency bands to reproduce the originalhand.

20.. A system for transmitting over a conductor waves having componentshigher than the practical upper frequency trans wave after transmissionwith a wave of said mission limit of said conductor, whichcomsinglefrequency to increase the component 10 prises means forcombining said waves with frequencies thereof to their original freawave of a single frequency to reduce the quency magnitudes.

component frequencies by' said single fre- In testimony'whereof, I havesigned my quency, 'means for transmitting said wave name to thisspecification this 26th day of of reduced fre uency components over saidJanuary, 1922.

conductor, an means for combining said HERMAN A AFFEL,

